There have been numerous applications of Magic Angle Spinning (MAS) for line narrowing in solid samples for more than two decades. The solid sample is usually contained in a hard ceramic rotor with press-fit turbine caps machined from high-strength high-modulus plastics such as polymides, although softer plastics such as Kel-F (poly-CCIF.sub.3, the highest strength non-protonated plastic available) are used when it is necessary to minimize .sup.1 H or .sup.13 C background signals--see for example, U.S. Pat. No. 5,508,615 by Doty et al (note the extensive list of typographical corrections). For high-temperature applications, the sample is usually contained in a soft boron-nitride cell inside a hard ceramic rotor, as disclosed by Doty et al in U.S. Pat. No. 5,202,633. The limited MAS applications to liquids prior to 1993 generally utilized viton o-rings in either ceramic or plastic turbine caps inside hard ceramic rotors, as shown by Daugaard et al in U.S. Pat. No. 4,739,270, although press-fit kel-f plugs inside ceramic rotors have also been used and found to seal well when the rotor inside diameter (D) is highly polished.
A number of recent publications describe extensive new applications of MAS to liquid and semi-solid samples--see for example, `A Comparison of NMR Spectra Obtained for Solid-Phase-Synthesis Resins Using Conventional High-Resolution, MAS, and HR-MAS Probes`, in J. Magn. Reson. Ser. A, vol. 119, pp. 65-75, 1996, by Paul A. Keifer et al, and `The influence of Resin Structure, Tether Length, and solvent upon the High-Resolution .sup.1 H NMR Spectra of Solid-Phase-Synthesis Resins`, in J. Org. Chem., vol. 61, pp. 1558-1559, 1996, by Paul A. Keifer. The referenced work utilizes a sample cell by Vrian NMRI (Palo Alto, Calif.), as shown in FIG. 1, capable of spinning between approximately 1000 and 3000 Hz. It uses a glass rotor tube 1 with a teflon plug 2 and polyimide conical drive turbine 3. While the seal is quite effective, the spinner design does not work well above 3 kHz, and the samples must be loaded through a syringe, which is not convenient for many types of semi-solids. A copending patent application discloses some methods of achieving the high B.sub.0 homogeneity in the spinner assembly needed for high resolution (HR) MAS with liquids. Another copending patent application describes novel lock coils for high resolution, another discloses novel NMR susceptibility plugs, and yet another discloses novel transverse coil geometries.
The semi-solids MAS applications stem largely from the fact that spinning a cylindrically symmetric sample at the magic angle averages susceptibility discontinuities to zero. Hence, high resolution may be obtained with magnetically inhomogeneous samples, such as tissues and semi-solids, and the discontinuity at the sample-cell boundary is inconsequential, obviating the need for susceptibility matching. This is particularly important for applications with limited samples.
The MAS o-ring seal fails (at its minor diameter) above a critical speed that is a function of sample density, o-ring density, o-ring compression, and diameter. For 5 mm rotors, the maximum speed is usually in the range of 5 to 8 kHz, corresponding to centrifugal sample fluid pressures in the range of 12 to 50 bar. On the other hand, solid polymeric plugs, such as teflon or kel-f, generally maintain a fluid-tight seal without o-rings up to the maximum speed the ceramic rotor will tolerate (often 12 to 20 kHz for 5 mm MAS rotors) as long as the fluid density is less than the density of the polymeric plug and the temperature is not far below room temperature. However, ceramic or glass MAS rotors with turbine caps and sealing closures of the prior art are somewhat difficult to load/unload and are quite expensive--typically in the range of $250 to $900 each.
Another approach in a variety of designs over the past 40 years in an all-plastic rotor--see, for example, Bartuska et al in U.S. Pat. No. 4,511,841. Often the parts have been threaded for a screw-together cell. Other designs have used press-fit parts or sealed glass tubes inside ceramic rotors. None of these approaches is satisfactory for sealing liquids in a high speed spinner. First of all, the modulus of a kel-f rotor is too low to maintain the required precision for high-speed spinning. But even before this spinning limitation is reached, the cells leak, as the outer plastic shell stretches away from what ever type of seal is designed. Spinning of 5 mm kel-f rotors inside stiff gas bearings is generally limited to less than 5 kHz, but sealing often fails below 3 kHz. Numerous plastics are available with much higher strength and modulus and thus permit higher spinning speeds--at least up to 10 kHz. However, sealing is often more difficult because of the lack of compliance, and, because of their backgrounds, such plastics are useless for the primary application of HR NMR--proton NMR.
An important new application of HR MAS is in combinatorial chemistry, where it is often desirable to maintain thousands of prepared samples that can easily be loaded into an MAS probe, spun up to at least 6 kHz (sometimes even 18 kHz is needed), easily removed without contamination, and stored for years in sealed cells for later analysis by other methods. The sample cells disclosed herein satisfy this and other needs and are more than an order of magnitude less expensive than the prior art ceramic cells.